Thermal interface materials (TIM) are used to minimize the contact thermal resistance between a heat source and a heat sink. It is widely applied in electronic and other industries where heat removal from chips or processors is critical since operation of integrated circuits at elevated temperatures is a major cause of failure for electronic devices. Such thermal management is becoming more and more important with the rapidly increasing functions and hence power densities of advanced electronics. Generated heat needs to be transferred or dissipated to a heat, sink in order to maintain an appropriate operating temperature.
When two solid surfaces, such as a heat source and a heat sink, are put together, however, the real contact area between them is limited due to the fact that the two surfaces are not completely flat, only a very small portion of the apparent surface area is actually in contact. As a result, thermal transfer between these mating surfaces is very limited as well, causing a notable temperature difference at the interface. The major role of TIMs is to fill the gap between the two mating surfaces and increase the heat transfer between them. The major requirements for a TIM material are: high thermal conductivity, easily deformed by a small pressure to fill the void between the contacting surfaces, good wetting and affinity with the two contacting surfaces, ability to form a layer with minimal thickness, mechanically stable, not easy to leak out, good thermal cycle life, and, easy to apply.
Traditional TIMs include greases, pads, gels, adhesives, solders, and phase change materials, etc. Most of them are made of a polymer or silicone matrix loaded with thermally conductive filler particles.
Thermal greases are a form of thick paste composed of thermally conductive filler dispersed in silicone or hydrocarbon oil. The filler can toe metallic, ceramic, or carbonaceous materials. Metal-based thermal greases often employ silver, copper, or aluminum particles. They usually have good thermal conductivity, but may suffer high cost.
Additionally, they are electrically conductive which may limit their applications without an additional electrically insulating material to go with them. Ceramic-based thermal pastes typically use conductive ceramic particles, such as beryllium oxide, aluminum nitride, aluminum oxide, zinc oxide, and silica as the filler. They usually have good thermal conductivity and low cost. Carbon-based thermal greases are relatively new. Good fillers include carbon nanotube (CNT) and carbon nano fibers (CNF). In general, thermal greases have high thermal conductivity, thin bond line thickness (BLT) with minimal pressure, low viscosity to fill the voids between mating surfaces, and no need to be cured. However, thermal grease is susceptible to grease pump-out and messy to apply. The pump-out is typically caused by mismatched coefficients of thermal expansion (CTE) of the mating surfaces, which could force the TIM to flow out of the interface by alternately squeezing and releasing the system during thermal cycling.
Thermal pads are a group of TIMs in the form of pad. They typically consist of an elastomer matrix such as silicone rubber and thermally conductive fillers such as boron nitride, alumina, or sine oxide. The material is often made into a soft pad that can be conformable to the mating surfaces upon compression, they are easily handled and applied, less susceptible to pump-out, and can serve as a vibration damper. Their major drawbacks include the need for high contact pressure and lower thermal conductivity and higher costs than thermal greases.
Thermal gels typically consist of silicone (or olefin) polymers with low cross-lint density loaded with a thermally conductive filler, either ceramic or metallic. The silicone has low modulus of elasticity, good wetting characteristics and high thermal stability. The materials are like greases but can be cured. They have relatively decent thermal conductivity, good wetting characteristics, easy to conform to mating surfaces, and are less susceptible to pump out. However, they need to be cured and may delaminate during thermal cycling.
Thermal adhesive; are a type of thermally conductive glue normally consisting of an adhesive resin and a thermally conductive filler. An example is silver particles dispersed in a cured epoxy matrix. Such TIMs eliminate the need for mechanical attachment of permanent pressure and are easy to apply. They are not susceptible to pump-out and can conform to the mating surfaces. However, they need to be cured and there is a risk of delamination during usage.
A phase change material (PCM) is a substance with a high heat of fusion which is capable of storing or releasing a large amount of energy upon melting or solidifying. The phase change thermal, interface materials are typically made of suspended particles of high thermal conductivity and a base material. Examples include conductive metal oxide particles dispersed in an organic matrix such as fully refined paraffin, a polymer, a co-polymer, or a mixture of the three. At room temperatures, they are similar to thermal pads. When heated, to a certain temperature, normally >50° C., they change to semi-solids or liquids to fill the void between mating surfaces. They solidify again when the temperature drops below the transition temperature. PCM is less susceptible to pump-out and its application is easier than grease. It also does not need to cure and there is no delamination concern. The major drawbacks are their lower conductivity as compared to that of grease and a pressure is required as well.
The thermal conductivity of commercially available TIMs is around 5 W/mK which is considerably lower than those of the typical mating surfaces. As a result, there has been a growing interest in searching for better TIMs, especially more effective fillers. Advanced carbon-based nano materials such as carbon nanotube, graphene, and graphene nanoplatelets are promising candidates due to their high intrinsic thermal conductivities. For example, the thermal conductivity of single-wall carbon nanotube (CNT) is in the range of 3000-5000 W/mK at room temperature whereas that of graphene is even higher. While CNT has received considerable attention for TIM applications, it has yet to foe commercially successful due to both performance and manufacturing cost issues. Early work focused on dispersing CNT randomly and the results have been less satisfactory. Recently, attention has shifted to the vertical alignment of CNT and reduction of boundary resistance at the interface between CNT and two mating surfaces. But, it will be a challenge for such a technology to be used for mass application due to its high processing costs.
Recently, graphene became a new focus for advanced thermal management solutions due to its high thermal conductivity. Graphene has a unique thermal property: it has an extremely high in-plane thermal conductivity but the through-plane conductivity of graphene is at least two orders of magnitude lower. The high in-plane thermal conductivity results from the covalent sp2 bonding between carbon atoms whereas the poor through-plane thermal conductivity is mainly due to weak van der Waals coupling in that direction. The thermal conductivity of a suspended monolayer graphene was reported to be about 5000 W/mK when measured by an optical method from shift in Raman G band. It's for this reason attempts have been made by many investigators to incorporate graphene or graphene nanoplatelets in various materials or forms for thermal applications including sheet products for thermal dissipation and graphene-based pastes/adhesives for thermal interface heat transfer.
For example, the inventors herein have developed a graphene-based sheet product (XG Leaf B) that can be used for spreading heats from a heat source to a heat sink. The material has a high in-plane thermal conductivity of >500 W/mK and a low through-plane conductivity of <5 W/mK. This material utilizes the 2-dimensional and anisotropic features of graphene nanoplatelets so that heat is dissipated laterally away from heat source instead of transferring through to other parts of electronic devices. For some other applications, however, heat needs to be transferred across two mating surfaces of a heat source and a heat sink. For example, FIG. 1 shows an application of thermal interface material in a LED lighting device. Currently, silver-based solder paste is used to transfer heat away from LED chip to a heat sink. There are several disadvantages with this thermal management solution. First, the paste needs to be cured at a temperature that can easily cause damage to the chips. Second, once cured, it is very difficult to be taken apart. When one chip fails, the entire unit has to be replaced. Third, silver-based paste is expensive. As a result, it is desired to replace the solder paste with a thin grease, gel, or tape with a high through-plane thermal conductivity and low cost. Some other applications require the TIM to be in the form of an adhesive, pad, phase change materials, and the like. It was under this circumstance that following inventions were conceived.
Graphene and graphene-based materials have been used in thermal interface materials as a filler as found in WO2015/103435, US2014/328024, and US2014/120399. Due to their 2-dimensional, nature, however, graphene sheets or graphene nanoplatelets tend to align or orient parallel, to the thermal interface, especially under pressure. As a result, the effect in enhancing the thru-plane thermal conductivity is substantially diminished. Therefore, it is imperative to establish a thermal pathway that can effectively conduct heat in the through plane direction. The instant invention has unique distinctions over the prior art.
WO2015/103435, deals with a method of aligning graphene flakes perpendicular to the mating substrate using magnetic functionalization and magnetic fields. This requires expensive specialized equipment to generate the magnetic fields. Additionally, the graphene alignment may decrease over time, in a fluid system, once the magnetic field is no longer applied. In the instant invention, graphene, graphene nanoplatelets, or toher thermally conductive materials such as boron nitride platelet can be coated or anchored on the surface of fillers. The partial graphene platelet alignment perpendicular to the mating substrates is an inherent property of a TIM made with graphene or another coated filler, and will remain stable. Such a TIM can be processed using standard industry equipment and methods and still get the benefit of aligned, graphene and other thermally conductive platelets.
US2014/120399 describes the thermal benefits of adding graphene to a matrix for use as a TIM, but makes no mention of the problem of platelet alignment encountered in the thin bond lines used in practical TIM applications. Our instant invention addresses that alignment problem.
US2013/0221268 describes a thermal paste using graphene platelets in conjunction with other filler materials to create a 3D conductive network. However, by coating the graphene platelets onto the other fillers without significantly damaging their structure the instant invention achieves similar thermal conductivity improvement with greatly reduced viscosity, resulting in a superior product for handling and thermal resistance.
U.S. Pat. No. 7,886,813 describes a TIM material with filler particles coated with high thermal conductivity coatings. The coatings are metals and the use of a graphite or a sheet material to coat the fillers is not contemplated.
U.S. 2014/025578 describes 3 process for coating particles with graphene. Use of such particles for heat transfer such as in a TIM is not contemplated.